Along with the development of optoelectronic technology, sensing devices have been broadly applied to different technical fields, such as optical image sensors, digital radiography sensors (DRS), and touch screen sensors. A sensing device is mainly composed of an active sensing array, and the active sensing array includes a plurality of pixel circuits.
In order to meet the high security demand, a sensing device is designed to have high sensitivity, so that the pixel circuit can detect incident energy of low-dose radiation. Besides, in order to meet the demand for high resolution, the area of a single pixel circuit is relatively reduced, and accordingly the photosensitive area of a sensing element in the pixel circuit is reduced. It should be noted that the intensity of an electric signal generated by the pixel circuit is drastically reduced with the decrease in the radiation dose of the incident energy and the photosensitive area of the sensing element.
The characteristic differences between transistors in the pixel circuit become obvious and can not be neglected due to the weak electric signal. For example, the threshold voltage of each transistor changes with process variation. Besides, when the transistors have different threshold voltages, different sensing results will be produced even though the same incident energy is supplied to each pixel circuit. As a result, sensing errors will be produced.
To resolve the problem mentioned above, some techniques for compensating the threshold voltages of the transistors in a pixel circuit have been developed, such as the “Investigation of gain non-uniformities in the two TFT, current programmed amorphous silicon active pixel sensor for fluoroscopy, chest radiography and mammography tomosynthesis applications” published by N. Safavian et al. in volume 7622 of proceedings of SPIE and the “TFT active image sensor with current-mode readout circuit for digital x-ray fluoroscopy” published by N. Safavian et al. in volume 5969 of proceedings of SPIE.
The technique provided by N. Safavian et al. in the volume 7622 of the proceedings of SPIE cannot provide regional compensation or one-time compensation to the pixel circuits (i.e., cannot effectively compensate each pixel circuit in an active sensing array). Additionally, the pixel circuit disclosed by N. Safavian in the volume 5969 of the proceedings of SPIE has a 3-transistor (3T) structure. Thus, the pixel circuit has a large layout area therefore can only be applied to low-resolution sensing devices.
Thereby, how to eliminate the sensing errors caused by the characteristic differences between different elements when the resolution and sensitivity of a sensing device are improved has become a major subject in the design of sensing devices.